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Network Working Group                                            H. Chen
Internet-Draft                                                  D. Cheng
Intended status: Standards Track                     Huawei Technologies
Expires: March 24, 2019                                           M. Toy
                                                                 Verizon
                                                                 Y. Yang
                                                                     IBM
                                                      September 20, 2018


                         LS Flooding Reduction
                  draft-cc-ospf-flooding-reduction-04

Abstract

   This document proposes an approach to flood link states on a topology
   that is a subgraph of the complete topology per underline physical
   network, so that the amount of flooding traffic in the network is
   greatly reduced, and it would reduce convergence time with a more
   stable and optimized routing environment.  The approach can be
   applied to any network topology in a single area.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 24, 2019.







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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Conventions Used in This Document . . . . . . . . . . . . . .   4
   4.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Flooding Topology . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Construct Flooding Topology . . . . . . . . . . . . . . .   5
     5.2.  Backup for Flooding Topology Split  . . . . . . . . . . .   7
   6.  Extensions to OSPF  . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Extensions for Operations . . . . . . . . . . . . . . . .   8
     6.2.  Extensions for Centralized Mode . . . . . . . . . . . . .   9
       6.2.1.  Message for Flooding Topology . . . . . . . . . . . .   9
       6.2.2.  Encodings for Backup Paths  . . . . . . . . . . . . .  16
       6.2.3.  Message for Incremental Changes . . . . . . . . . . .  24
       6.2.4.  Leaders Selection . . . . . . . . . . . . . . . . . .  25
   7.  Extensions to IS-IS . . . . . . . . . . . . . . . . . . . . .  27
     7.1.  Extensions for Operations . . . . . . . . . . . . . . . .  27
     7.2.  Extensions for Centralized Mode . . . . . . . . . . . . .  27
       7.2.1.  TLV for Flooding Topology . . . . . . . . . . . . . .  27
       7.2.2.  Encodings for Backup Paths  . . . . . . . . . . . . .  28
       7.2.3.  TLVs for Incremental Changes  . . . . . . . . . . . .  29
       7.2.4.  Leaders Selection . . . . . . . . . . . . . . . . . .  30
   8.  Flooding Behavior . . . . . . . . . . . . . . . . . . . . . .  30
     8.1.  Nodes Perform Flooding Reduction without Failure  . . . .  30
       8.1.1.  Receiving an LS . . . . . . . . . . . . . . . . . . .  30
       8.1.2.  Originating an LS . . . . . . . . . . . . . . . . . .  31
       8.1.3.  Establishing Adjacencies  . . . . . . . . . . . . . .  31
     8.2.  An Exception Case . . . . . . . . . . . . . . . . . . . .  32
       8.2.1.  A Critical Failure  . . . . . . . . . . . . . . . . .  32
       8.2.2.  Multiple Failures . . . . . . . . . . . . . . . . . .  32
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  33
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  33



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     10.1.  OSPFv2 . . . . . . . . . . . . . . . . . . . . . . . . .  33
     10.2.  OSPFv3 . . . . . . . . . . . . . . . . . . . . . . . . .  35
     10.3.  IS-IS  . . . . . . . . . . . . . . . . . . . . . . . . .  36
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  36
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  36
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  36
     12.2.  Informative References . . . . . . . . . . . . . . . . .  37
   Appendix A.  Algorithms to Build Flooding Topology  . . . . . . .  37
     A.1.  Algorithms to Build Tree without Considering Others . . .  37
     A.2.  Algorithms to Build Tree Considering Others . . . . . . .  39
     A.3.  Connecting Leaves . . . . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  42

1.  Introduction

   For some networks such as dense Data Center (DC) networks, the
   existing Link State (LS) flooding mechanism is not efficient and may
   have some issues.  The extra LS flooding consumes network bandwidth.
   Processing the extra LS flooding, including receiving, buffering and
   decoding the extra LSs, wastes memory space and processor time.  This
   may cause scalability issues and affect the network convergence
   negatively.

   This document proposes an approach to minimize the amount of flooding
   traffic in the network.  Thus the workload for processing the extra
   LS flooding is decreased significantly.  This would improve the
   scalability, speed up the network convergence, stable and optimize
   the routing environment.

   This approach is also flexible.  It has multiple modes for
   computation of flooding topology.  Users can select a mode they
   prefer, and smoothly switch from one mode to another.  The approach
   is applicable to any network topology in a single area.  It is
   backward compatible.

2.  Terminology

   Flooding Topology:
       A sub-graph or sub-network of a given (physical) network topology
       that has the same reachability to every node as the given network
       topology, through which link states are flooded.

   critical link or interface on a flooding topology:
       A only link or interface among some nodes on the flooding
       topology.  When this link or interface goes down, the flooding
       topology will be split.

   critical node on a flooding topology:



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       A only node connecting some nodes on the flooding topology.  When
       this node goes down, the flooding topology will be split.

   backup path:
       A path or a sequence of links, when a critical link or node goes
       down, providing a connection to connect two parts of a split
       flooding topology.  When a critical node goes down, the flooding
       topology may be split into more than two parts.  In this case,
       two or more backup paths are needed to connect all the split
       parts into one.

   Remaining Flooding Topology:
       A topology from a flooding topology by removing the failed links
       and nodes from the flooding topology.

   LSA:
       A Link State Advertisement in OSPF.

   LSP:
       A Link State Protocol Data Unit (PDU) in IS-IS.

   LS:
       A Link Sate, which is an LSA or LSP.

3.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

4.  Problem Statement

   OSPF and IS-IS deploy a so-called reliable flooding mechanism, where
   a node must transmit a received or self-originated LS to all its
   interfaces (except the interface where an LS is received).  While
   this mechanism assures each LS being distributed to every node in an
   area or domain, the side-effect is that the mechanism often causes
   redundant LS, which in turn forces nodes to process identical LS more
   than once.  This results in the waste of link bandwidth and nodes'
   computing resources, and the delay of topology convergence.

   This becomes more serious in networks with large number of nodes and
   links, and in particular, higher degree of interconnection (e.g.,
   meshed topology, spine-leaf topology, etc.).  In some environments
   such as in data centers, the drawback of the existing flooding
   mechanism has already caused operational issues, including repeated
   and waves of flooding storms, chock of computing resources, slow




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   convergence, oscillating topology changes, instability of routing
   environment.

   One example is as shown in Figure 1, where Node 1, Node 2 and Node 3
   are interconnected in a mesh.  When Node 1 receives a new or updated
   LS on its interface I11, it by default would forward the LS to its
   interface Il2 and I13 towards Node 2 and Node 3, respectively, after
   processing.  Node 2 and Node 3 upon reception of the LS and after
   processing, would potentially flood the same LS over their respective
   interface I23 and I32 toward each other, which is obviously not
   necessary and at the cost of link bandwidth as well as both nodes'
   computing resource.

                                   |
                                   |I11
                                +--o---+
                                |Node 1|
                                +-o--o-+
                             I12 /    \ I13
                                /      \
                            I21/        \I31
                         +----o-+   I32+-o----+
                         |Node 2|------|Node 3|
                         +------+I23   +------+

                                 Figure 1

5.  Flooding Topology

   For a given network topology, a flooding topology is a sub-graph or
   sub-network of the given network topology that has the same
   reachability to every node as the given network topology.  Thus all
   the nodes in the given network topology MUST be in the flooding
   topology.  All the nodes MUST be inter-connected directly or
   indirectly.  As a result, LS flooding will in most cases occur only
   on the flooding topology, that includes all nodes but a subset of
   links.  Note even though the flooding topology is a sub-graph of the
   original topology, any single LS MUST still be disseminated in the
   entire network.

5.1.  Construct Flooding Topology

   Many different flooding topologies can be constructed for a given
   network topology.  A chain connecting all the nodes in the given
   network topology is a flooding topology.  A circle connecting all the
   nodes is another flooding topology.  A tree connecting all the nodes
   is a flooding topology.  In addition, the tree plus the connections




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   between some leaves of the tree and branch nodes of the tree is a
   flooding topology.

   The following parameters need to be considered for constructing a
   flooding topology:

   o  Number of links: The number of links on the flooding topology is a
      key factor for reducing the amount of LS flooding.  In general,
      the smaller the number of links, the less the amount of LS
      flooding.

   o  Diameter: The shortest distance between the two most distant nodes
      on the flooding topology is a key factor for reducing the network
      convergence time.  The smaller the diameter, the less the
      convergence time.

   o  Redundancy: The redundancy of the flooding topology means a
      tolerance to the failures of some links and nodes on the flooding
      topology.  If the flooding topology is split by some failures, it
      is not tolerant to these failures.  In general, the larger the
      number of links on the flooding topology is, the more tolerant the
      flooding topology to failures.

   There are many different ways to construct a flooding topology for a
   given network topology.  A few of them are listed below:

   o  Central Mode: One node in the network builds a flooding topology
      and floods the flooding topology to all the other nodes in the
      network (This seems not good.  Flooding the flooding topology may
      increase the flooding.  The amount of traffic for flooding the
      flooding topology should be minimized.);

   o  Distributed Mode: Each node in the network automatically
      calculates a flooding topology by using the same algorithm (No
      flooding for flooding topology);

   o  Static Mode: Links on the flooding topology are configured
      statically.

   Note that the flooding topology constructed by a node is dynamic in
   nature, that means when the base topology (the entire topology graph)
   changes, the flooding topology (the sub-graph) MUST be re-computed/
   re-constructed to ensure that any node that is reachable on the base
   topology MUST also be reachable on the flooding topology.

   For reference purpose, some algorithms that allow nodes to
   automatically compute flooding topology are elaborated in Appendix A.




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   However, this document does not attempt to standardize how a flooding
   topology is established.

5.2.  Backup for Flooding Topology Split

   It is hard to construct a flooding topology that reduces the amount
   of LS flooding greatly and is tolerant to multiple failures.  To get
   around this, we can compute and use backup paths for a critical link
   and node on the flooding topology.  Using backup paths may also speed
   up convergence when the link and node fail.

   When a critical link on the flooding topology fails, the flooding
   topology without the critical link (i.e., the remaining flooding
   topology) is split into two parts.  A backup path for the critical
   link connects the two parts into one.  Through the backup path and
   the remaining flooding topology, an LS can be flooded to every node
   in the network.  The combination of the backup path and the flooding
   topology is tolerant to the failure of the critical link.

   When a critical node on the flooding topology goes down, the flooding
   topology without the critical node and the links attached to the node
   (i.e., the remaining flooding topology) is split into two or more
   parts.  One or more backup paths for the critical node connects the
   split parts into one.  Through the backup paths and the remaining
   flooding topology, an LS can be flooded to every live node in the
   network.  The combination of the backup paths and the flooding
   topology is tolerant to the failure of the critical node.

   In addition to the backup paths for a critical link and node, backup
   paths for every non critical link and node on the flooding topology
   can be computed.  When the failures of multiple links and nodes on
   the flooding topology happen, through the remaining flooding topology
   and the backup paths for these links and nodes, an LS can be flooded
   to every live node in the network.  The combination of the backup
   paths and the flooding topology is tolerant to the failures of these
   links and nodes.  If there are other failures that break the backup
   paths, an LS can be flooded to every live node by the traditional
   flooding procedure.

   In a centralized mode, the leader computes the backup paths and
   floods them to all the other nodes.  In a distributed mode, every
   node computes the backup paths.

6.  Extensions to OSPF

   The extensions to OSPF comprises two parts: one part is for
   operations on flooding reduction, the other is specially for
   centralized mode flooding reduction.



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6.1.  Extensions for Operations

   A new TLV is defined in OSPF RI LSA [RFC7770].  It contains
   instructions about flooding reduction, which is called Flooding
   Reduction Instruction TLV or Instruction TLV for short.  This TLV is
   originated from only one node at any time.

   The format of a Flooding Reduction Instruction TLV is as follows.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      INST-TLV-Type (TBD1)     |          TLV-Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  OP | MOD |   Algorithm   |    Reserved (MUST be zero)  |  NL |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    sub TLVs (optional)                        ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Flooding Reduction Instruction TLV


   A OP field of three bits is defined in the TLV.  It may have a value
   of the followings.

   o  0x001 (R): Perform flooding Reduction, which instructs the nodes
      in a network to perform flooding reduction.

   o  0x010 (N): Roll back to Normal flooding, which instructs the nodes
      in a network to roll back to perform normal flooding.

   When any of the other values is received, it is ignored.

   A MOD field of three bits is defined in the TLV and may have a value
   of the followings.

   o  0x001 (C): Central Mode, which instructs 1) the nodes in a network
      to select leaders (primary/designated leader, secondary/backup
      leader, and so on); 2) the leaders in a network to compute a
      flooding topology and the primary leader to flood the flooding
      topology to all the other nodes in the network; 3) every node in
      the network to receive and use the flooding topology originated by
      the primary leader.

   o  0x010 (D): Distributed Mode, which instructs every node in a
      network to compute and use its own flooding topology.





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   o  0x011 (S): Static Mode, which instructs every node in a network to
      use the flooding topology statically configured on the node.

   When any of the other values is received, it is ignored.

   An Algorithm field of eight bits is defined in the TLV to instruct
   the leader node in central mode or every node in distributed mode to
   use the algorithm indicated in this field for computing a flooding
   topology.

   A NL field of three bits is defined in the TLV, which indicates the
   number of leaders to be selected when Central Mode is used.  NL set
   to 2 means two leaders (a designated/primary leader and a backup/
   secondary leader) to be selected for an area, and NL set to 3 means
   three leaders to be selected.  When Central Mode is not used, The NL
   field is not valid.

   Some optional sub TLVs may be defined in the future, but none is
   defined now.

6.2.  Extensions for Centralized Mode

6.2.1.  Message for Flooding Topology

   A flooding topology can be represented by the links in the flooding
   topology.  For the links between a local node and a number of its
   adjacent (or remote) nodes, we can encode the local node in a way,
   and encode its adjacent nodes in the same way or another way.  After
   all the links in the flooding topology are encoded, the encoded links
   can be flooded to every node in the network.  After receiving the
   encoded links, every node decodes the links and creates and/or
   updates the flooding topology.

   For every node in an area, we may use an index to represent it.
   Every node in an area may order the nodes in a rule, which generates
   the same sequence of the nodes on every node in the area.  The
   sequence of nodes have the index 0, 1, 2, and so on respectively.
   For example, every node orders the nodes by their router IDs in
   ascending order.

6.2.1.1.  Links Encoding

   A local node can be encoded in two parts: encoded node index size
   indication (ENSI) and compact node index (CNI).  ENSI value plus a
   number (e.g., 9) gives the size of compact node index.  For example,
   ENSI = 0 indicates that the size of CNIs is 9 bits.  In the figure
   below, Local node LN1 is encoded as ENSI=0 using 3 bits and CNI=LN1's
   Index using 9 bits.  LN1 is encoded in 12 bits in total.



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     0 1 2 3 4 5 6 7 8
    +-+-+-+-+-+-+-+-+-+
    |0 0 0|              ENSI (3 bits) [9 bits CNI]
    +-+-+-+-+-+-+-+-+-+
    | LN1 Index Value |  CNI  (9 bits)
    +-+-+-+-+-+-+-+-+-+

           An Example of Local Node Encoding


   The adjacent nodes can be encoded in two parts: Number of Nodes (NN)
   and compact node indexes (CNIs).  The size of CNIs is the same as the
   local node.  For example, three adjacent nodes RN1, RN2 and RN3 are
   encoded below in 30 bits (i.e., 3.75 bytes).

     0 1 2 3 4 5 6 7 8
    +-+-+-+-+-+-+-+-+-+
    |0 1 1|             NN (3 bits)   [3 adjacent nodes]
    +-+-+-+-+-+-+-+-+-+
    |   RN1's Index   | CNI (9 bits) for RN1
    +-+-+-+-+-+-+-+-+-+
    |   RN2's Index   | CNI (9 bits) for RN2
    +-+-+-+-+-+-+-+-+-+
    |   RN3's Index   | CNI (9 bits) for RN3
    +-+-+-+-+-+-+-+-+-+

        An Example of Adjacent Nodes Encoding


   The links between a local node and a number of its adjacent (or
   remote) nodes can be encoded as the local node followed by the
   adjacent nodes.  For example, three links between local node LN1 and
   its three adjacent nodes RN1, RN2 and RN3 are encoded below in 42
   bits (i.e., 5.25 bytes).

















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     0 1 2 3 4 5 6 7 8
    +-+-+-+-+-+-+-+-+-+                            _
    |0 0 0|             ENSI (3 bits) [9 bits CNI]  |
    +-+-+-+-+-+-+-+-+-+                             } Encoding for
    | LN1 Index Value | CNI (9 bits) for LN1       _| Local Node LN1
    +-+-+-+-+-+-+-+-+-+                            _
    |0 1 1|             NN (3 bits) [3 nodes]       |
    +-+-+-+-+-+-+-+-+-+                             | Encoding for
    |   RN1's Index   | CNI (9 bits) for RN1        | 3 adjacent nodes
    +-+-+-+-+-+-+-+-+-+                             } RN1, RN2, RN3
    |   RN2's Index   | CNI (9 bits) for RN2        | of LN1
    +-+-+-+-+-+-+-+-+-+                             |
    |   RN3's Index   | CNI (9 bits) for RN3       _|
    +-+-+-+-+-+-+-+-+-+

                         An Example of Links Encoding


   For a flooding topology computed by a leader of an area, it may be
   represented by all the links on the flooding topology.  A Type-
   Length-Value (TLV) of the following format for the links encodings
   can be included in an LSA to represent the flooding topology (FT) and
   flood the FT to every node in the area.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      FTLK-TLV-Type (TBD2)     |          TLV-Length           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 1 to its adjacent Nodes)       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 2 to its adjacent Nodes)       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                                                               :
    :                                                               :
                         Flooding Topology Links TLV


   Note that a link between a local node LN and its adjacent node RN can
   be encoded once and as a bi-directional link.  That is that if it is
   encoded in a Links Encoding from LN to RN, then the link from RN to
   LN is implied or assumed.

   For OSPFv2, an Opaque LSA of a new opaque type (TBD3) containing a
   Flooding Topology Links TLV is used to flood the flooding topology
   from the leader of an area to all the other nodes in the area.





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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            LS age             |     Options   | LS Type = 10  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | FT-Type(TBD3) |                   Instance ID                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Advertising Router                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      LS Sequence Number                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         LS checksum           |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                Flooding Topology Links TLV                    ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     OSPFv2: Flooding Topology Opaque LSA


   For OSPFv3, an area scope LSA of a new LSA function code (TBD4)
   containing a Flooding Topology Links TLV is used to flood the
   flooding topology from the leader of an area to all the other nodes
   in the area.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            LS age             |1|0|1|       FT-LSA (TBD4)     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Link State ID                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Advertising Router                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      LS Sequence Number                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         LS checksum           |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                Flooding Topology Links TLV                    ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        OSPFv3: Flooding Topology  LSA


   The U-bit is set to 1, and the scope is set to 01 for area-scoping.







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6.2.1.2.  Block Encoding

   Block encoding uses a single structure to encode a block (or part) of
   topology, which can be a block of links in a flooding topology.  It
   can also be all the links in the flooding topology.  It starts with a
   local node LN and its adjacent (or remote) nodes RNi (i = 1, 2, ...,
   n), and can be considered as an extension to the links encoding.

   The encoding of links between a local node and its adjacent nodes
   described in Section 6.2.1.1 is extended to include the links
   attached to the adjacent nodes.

   The encoding for the adjacent nodes is extended to include Extending
   Flags (E Flags for short) between the NN (Number of Nodes) field and
   the CNIs (Compact Node Indexes) for the adjacent nodes.  The length
   of the E Flags field is NN bits.  The following is an example
   encoding of the adjacent nodes with E Flags of 3 bits, which is the
   value of the NN (the number of adjacent nodes).

     0 1 2 3 4 5 6 7 8
    +-+-+-+-+-+-+-+-+-+
    |0 1 1|             NN (3 bits)   [3 adjacent nodes]
    +-+-+-+
    |1 0 1|             E Flags [NN=3 bits]
    +-+-+-+-+-+-+-+-+-+
    |   RN1's Index   | CNI (9 bits) for RN1
    +-+-+-+-+-+-+-+-+-+
    |   RN2's Index   | CNI (9 bits) for RN2
    +-+-+-+-+-+-+-+-+-+
    |   RN3's Index   | CNI (9 bits) for RN3
    +-+-+-+-+-+-+-+-+-+

    An Example of Adjacent Nodes with E Flags Encoding


   There is a bit flag (called E flag) in the E Flags field for each
   adjacent node.  The first bit (i.e., the most significant bit) in the
   E Flags field is for the first adjacent node (e.g., RN1), the second
   bit is for the second adjacent node (e.g., RN2), and so on.  The E
   flag for an adjacent node RNi set to one indicates that the links
   attached to the adjacent node RNi are included below.  The E flag for
   an adjacent node RNi set to zero means that no links attached to the
   adjacent node RNi are included below.

   The links attached to the adjacent node RNi are represented by the
   RNi as a local node and the adjacent nodes of RNi.  The encoding for
   the adjacent nodes of RNi is the same as that for the adjacent nodes




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   of a local node.  It consists of an NN field of 3 bits, E Flags field
   of NN bits, and CNIs for the adjacent nodes of RNi.

   The following is an example of a block encoding for a block (or part)
   of flooding topology below.

                             o LN1
                           / | \
                         /       \
                       /     |     \
                     o RN1   o RN2   o RN3
                   /               /   \
                 /               /       \
               /               /           \
              o RN11          o RN31        o RN32

             An Example Block of Flooding Topology


   It represents 6 links: 3 links between local node LN1 and its 3
   adjacent nodes RN1, RN2 and RN3; 1 link between RN1 as a local node
   and its 1 adjacent node RN11; and 2 links between RN3 as a local node
   and its 2 adjacent nodes RN31 and RN32.

   It starts with the encoding of the links between local node LN1 and 3
   adjacent nodes RN1, RN2 and RN3 of the local node LN1.  The encoding
   for the local node LN1 is the same as that for a local node described
   in Section 6.2.1.1.  The encoding for 3 adjacent nodes RN1, RN2 and
   RN3 of local node LN1 comprises an NN field of 3 bits with value of
   3, E Flags field of NN = 3 bits, and the indexes of adjacent nodes
   RN1, RN2 and RN3.




















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     0 1 2 3 4 5 6 7 8
    +-+-+-+-+-+-+-+-+-+                            _
    |0 0 0|             ENSI (3 bits) [9 bits CNI]  |
    +-+-+-+-+-+-+-+-+-+                             } Encoding for
    | LN1 Index Value | CNI  (9 bits)              _| Local Node LN1
    +-+-+-+-+-+-+-+-+-+                            _
    |0 1 1|             NN(3 bits)[3 adjacent nodes]|
    +-+-+-+                                         |
    |1 0 1|             E Flags [NN=3 bits]         | Encoding for
    +-+-+-+-+-+-+-+-+-+                             | 3 adjacent nodes
    |   RN1's Index   | CNI (9 bits) for RN1        } (RN1, RN2, RN3)
    +-+-+-+-+-+-+-+-+-+                             | of LN1
    |   RN2's Index   | CNI (9 bits) for RN2        |
    +-+-+-+-+-+-+-+-+-+                             |
    |   RN3's Index   | CNI (9 bits) for RN3       _|
    +-+-+-+-+-+-+-+-+-+                            _
    |0 0 1|             NN (3 bits)[1 adjacent node]|
    +-+-+-+                                         | Encoding for
    |0|                 E Flags [NN=1 bit]          } 1 adjacent node
    +-+-+-+-+-+-+-+-+-+                             | (RN11)
    |   RN11's Index  | CNI (9 bits) for RN11      _| of RN1
    +-+-+-+-+-+-+-+-+-+                            _
    |0 1 0|             NN(3 bits)[2 adjacent nodes]|
    +-+-+-+                                         |
    |0 0|               E Flags [NN=2 bits]         | Encoding for
    +-+-+-+-+-+-+-+-+-+                             } 2 adjacent nodes
    |   RN31's Index  | CNI (9 bits) for RN31       | (RN31, RN32)
    +-+-+-+-+-+-+-+-+-+                             | of RN3 as a
    |   RN32's Index  | CNI (9 bits) for RN32       | local node
    +-+-+-+-+-+-+-+-+-+                            _|

                    An Example of Block Encoding


   The first E flag in the encoding for adjacent nodes RN1, RN2 and RN3
   is set to one, which indicates that the links between the first
   adjacent node RN1 as a local node and its adjacent nodes are included
   below.  In this example, 1 link between RN1 and its adjacent node
   RN11 is represented by the encoding for the adjacent node RN11 of RN1
   as a local node.  The encoding for 1 adjacent node RN11 consists of
   an NN field of 3 bits with value of 1, E Flags field of NN = 1 bits,
   and the index of adjacent node RN11.  The size of the index of RN11
   is the same as that of local node LN1 indicated by the ENSI in the
   encoding for local node LN1.

   The second E flag in the encoding for adjacent nodes RN1, RN2 and RN3
   is set to zero, which indicates that no links between the second




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   adjacent node RN2 as a local node and its adjacent nodes are included
   below.

   The third E flag in the encoding for adjacent nodes RN1, RN2 and RN3
   is set to one, which indicates that the links between the third
   adjacent node RN3 as a local node and its adjacent nodes are included
   below.  In this example, 2 links between RN3 and its 2 adjacent nodes
   RN31 and RN32 are represented by the encoding for the adjacent nodes
   RN31 and RN32 of RN3 as a local node.  The encoding for 2 adjacent
   nodes RN31 and RN32 consists of an NN field of 3 bits with value of
   2, E Flags field of NN = 2 bits, and the indexes of adjacent nodes
   RN31 and RN32.  The size of the index of RN31 and RN32 is the same as
   that of local node LN1 indicated by the ENSI in the encoding for
   local node LN1.

   The block encoding may be used in the place of the links encoding in
   Section 6.2.1.1 for more efficiency.  That is that it may be used in
   a Flooding Topology Links TLV.  Alternatively, a new TLV, which is
   similar to the Flooding Topology Links TLV, may be defined to contain
   a number of block encodings.

6.2.2.  Encodings for Backup Paths

   When the leader of an area computes a flooding topology, it may
   compute a backup path or multiple backup paths for a critical link on
   the flooding topology.  When the critical link fails, a link state
   can be distributed to every node in the area through one backup path
   and other links on the flooding topology.  In addition, it may
   compute a backup path or multiple backup paths for a node.  When the
   node fails, a link state can be distributed to the other nodes in the
   area through the backup paths and the links on the flooding topology.

   This section describes two encodings for backup paths: separated
   encoding and integrated one.  In the former, backup paths are encoded
   in a new message, where the message for the flooding topology
   described in the previous section is required; In the latter, backup
   paths are integrated into the flooding topology links encoding, where
   one message contains the flooding topology and the backup paths.

6.2.2.1.  Message for Backup Paths

   Backup paths for a node (such as Node1) may be represented by the
   node index encoding and node backup paths encoding.  The former is
   similar to local node index encoding.  The latter has the following
   format.  It comprises a K flag (Key/Critical node flag) of 1 bit, a 3
   bits NNBP field (number of node backup paths), and each of the backup
   paths encoding, which consists of the path length PLEN of 4 bits
   indicating the length of the path (i.e., the number of nodes), and



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   the encoding of the sequence of nodes along the path such as
   encodings for nodes PN1, ..., PNn.  The encoding of every node may
   use the encoding of a local node, which comprises encoded node index
   size indication (ENSI) and compact node index (CNI).

 0 1 2 3 4 5 6 7 8
 +-+-+-+-+-+-+-+-+
 |K|                1 bit (K=1: Key/Critical Node, K=0: Normal Node)
 +-+-+-+                                                     _
 |NNBP |            3 bits (number of node backup paths)      |
 +-+-+-+-+                                   _                |
 |PLEN   |          4 bits (backup path len)  |               |
 +-+-+-+-+-+-+-+-+                            |               | Backup
 | PN1 Encoding  |  Variable bits             | One           } paths
 +-+-+-+-+-+-+-+-+                            } backup path   | for Node
 ~               ~                            | for Node      |
 +-+-+-+-+-+-+-+-+                            |               |
 | PNn Encoding  |  Variable bits            _|               |
 +-+-+-+-+-+-+-+-+                                            |
 //              //                                          _|

                    An Example of Node Backup Paths Encoding


   Another encoding of the sequence of nodes along the path uses one
   encoded node index size indication (ENSI) for all the nodes in the
   path.  Thus we have the following Node Backup Paths Encoding.

  0 1 2 3 4 5 6 7 8
  +-+-+-+-+-+-+-+-+
  |K|                1 bit (K=1: Key/Critical Node, K=0: Normal Node)
  +-+-+-+                                                    _
  |NNBP |            3 bits (number of node backup paths)     |
  +-+-+-+-+                                   _               |
  |PLEN   |          4 bits (backup path len)  |              |
  +-+-+-+-+                                    |              |
  |ENSI |            3 bits(Ix Bits Indication)|              | Backup
  +-+-+-+-+-+-+-+-+                            | One          } paths
  |   PN1 Index   |  #Bits indicated by ENSI   } backup path  | for Node
  +-+-+-+-+-+-+-+-+                            | for Node     |
  ~               ~                            |              |
  +-+-+-+-+-+-+-+-+                            |              |
  |   PNn Index   |  #Bits indicated by ENSI  _|              |
  +-+-+-+-+-+-+-+-+                                           |
  //              //                                         _|

                   Another Example of Node Backup Paths Encoding




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   A new TLV called Node Backup Paths TLV is defined below.  It may
   include multiple nodes and their backup paths.  Each node is
   represented by its index encoding, which is followed by its node
   backup paths encoding.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      NBP-TLV-Type (TBD5)      |          TLV-Length           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Node1 Index Enc|  Variable bits
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                 Node1 backup paths encoding                   :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Node2 Index Enc|  Variable bits
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                 Node2 backup paths encoding                   :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                                                             //
                            Node Backup Paths TLV


   The encoding for backup paths for a link (such as Link1) on the
   flooding topology consists of the link encoding such as Link1 Index
   Encoding and the link backup paths encoding.  The former is similar
   to local node encoding.  It contains encoded link index size
   indication (ELSI) and compact link index (CLI).  The latter has the
   following format.  It comprises a C flag (Critical link flag) of 1
   bit, a 2 bits NLB field (number of link backup paths), and each of
   the backup paths encoding, which consists of the path length PLEN of
   3 bits indicating the length of the path (i.e., the number of nodes),
   and the encoding of the sequence of nodes along the path such as
   encodings for nodes PN1, ..., PNm.  Note that two ends of a link
   (i.e., the local node and the adjacent/remote node of the link) are
   not needed in the path.  The encoding of every node may use the
   encoding of a local node, which comprises encoded node index size
   indication (ENSI) and compact node index (CNI).














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  0 1 2 3 4 5 6 7 8
  +-+-+-+-+-+-+-+-+
  |C|                1 bit (C=1: Critical Link, C=0: Normal Link)
  +-+-+                                                      _
  |NLB|              2 bits (number of link backup paths)     |
  +-+-+-+                                     _               |
  |PLEN |            3 bits (backup path len)  |              |
  +-+-+-+-+-+-+-+-+                            |              | Backup
  | PN1 Encoding  |  Variable bits             | One          } paths
  +-+-+-+-+-+-+-+-+                            } backup path  | for Link
  ~               ~                            | for Link     |
  +-+-+-+-+-+-+-+-+                            |              |
  | PNm Encoding  |  Variable bits            _|              |
  +-+-+-+-+-+-+-+-+                                           |
  //              //                                         _|

                  An Example of Link Backup Paths Encoding


   Another encoding of the sequence of nodes along the path uses one
   encoded node index size indication (ENSI) for all the nodes in the
   path.  Thus we have the following Link Backup Paths Encoding.

  0 1 2 3 4 5 6 7 8
  +-+-+-+-+-+-+-+-+
  |C|                1 bit (C=1: Critical Link, C=0: Normal Link)
  +-+-+                                                      _
  |NLB|              2 bits (number of link backup paths)     |
  +-+-+-+                                     _               |
  |PLEN |            3 bits (backup path len)  |              |
  +-+-+-+                                      |              |
  |ENSI |            3 bits(Ix Bits Indication)|              | Backup
  +-+-+-+-+-+-+-+-+                            | One          } paths
  |   PN1 Index   |  #Bits indicated by ENSI   } backup path  | for Link
  +-+-+-+-+-+-+-+-+                            | for Link     |
  ~               ~                            |              |
  +-+-+-+-+-+-+-+-+                            |              |
  |   PNm Index   |  #Bits indicated by ENSI  _|              |
  +-+-+-+-+-+-+-+-+                                           |
  //              //                                         _|

                  Another Example of Link Backup Paths Encoding


   A new TLV called Link Backup Paths TLV is defined below.  It may
   include multiple links and their backup paths.  Each link is
   represented by its index encoding, which is followed by its link
   backup paths encoding.



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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      LBP-TLV-Type (TBD6)      |          TLV-Length           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Link1 Index Enc|  Variable bits
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                 Link1 backup paths encoding                   :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Link2 Index Enc|  Variable bits
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                 Link2 backup paths encoding                   :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                                                             //
                            Link Backup Paths TLV


   For OSPFv2, an Opaque LSA of a new opaque type (TBD7), containing
   node backup paths TLVs and link backup paths TLVs, is used to flood
   the backup paths from the leader of an area to all the other nodes in
   the area.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            LS age             |     Options   | LS Type = 10  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | BP-Type(TBD7) |                   Instance ID                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Advertising Router                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      LS Sequence Number                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         LS checksum           |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    Link Backup Paths TLVs                     ~
      ~                    Node Backup Paths TLVs                     ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     OSPFv2: Backup Paths Opaque LSA


   For OSPFv3, an area scope LSA of a new LSA function code (TBD8),
   containing node backup paths TLVs and link backup paths TLVs, is used
   to flood the backup paths from the leader of an area to all the other
   nodes in the area.





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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            LS age             |1|0|1|      BP-LSA (TBD8)      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Link State ID                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Advertising Router                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      LS Sequence Number                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         LS checksum           |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    Link Backup Paths TLVs                     ~
      ~                    Node Backup Paths TLVs                     ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        OSPFv3: Backup Paths LSA


   The U-bit is set to 1, and the scope is set to 01 for area-scoping.

6.2.2.2.  Backup Paths in Links TLV

   A local node and its backup paths can be encoded in the following
   format.  It is the local node (such as local node LN1) encoding
   followed by the local node backup paths encoding, which is the same
   as the node backup paths encoding described in Section 6.2.2.1.

     0 1 2 3 4 5 6 7 8
    +-+-+-+-+-+-+-+-+-+                         _
    |ENSI |             3 bits(#bits indication) |
    +-+-+-+-+-+-+-+-+-+                          } Local Node LN1
    | LN1 Index Value | #bits indicated by ENSI _| Encoding
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :              Local node LN1 backup paths encoding             :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Local Node with Backup Paths Encoding


   A adjacent node and its backup paths can be encoded in the following
   format.  It is the adjacent node (such as adjacent node RN10) index
   value followed by the adjacent node backup paths encoding, which is
   the same as the node backup paths encoding described in
   Section 6.2.2.1.





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    +-+-+-+-+-+-+-+-+-+
    |RN10 Index Value |  (#bits indicated by ENSI)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :             adjacent node RN10 backup paths encoding          :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Adjacent Node with Backup Paths Encoding


   The links between a local node and a number of its adjacent nodes,
   the backup paths for each of the nodes, and the backup paths for each
   of the links can be encoded in the following format.  It is called
   Links from Node with Backup Paths Encoding.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :              Local Node with backup paths encoding            :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | NN  |   Number of adjacent Nodes (i.e., Number of links)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :              Adjacent Node 1 with backup paths encoding       :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                   Link1 backup paths Encoding                 :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :              Adjacent Node 2 with backup paths encoding       :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                   Link2 backup paths Encoding                 :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |

                Links from Node with Backup Paths Encoding


   A new TLV called Links with Backup Paths TLV is defined below.  It
   includes a number of Links from Node with Backup Paths Encodings
   described above.  This TLV contains both the flooding topology and
   the backup paths for the links and nodes on the flooding topology.













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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    LNSBP-TLV-Type (TBD9)      |          TLV-Length           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :          Links from Node 1 with backup paths encoding         :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :          Links from Node 2 with backup paths encoding         :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                                                               :
    :                                                               :
                      Links with Backup Paths TLV


   For OSPFv2, an Opaque LSA of a new opaque type (TBDa), called
   Flooding Topology with Backup Paths (FTBP) Opaque LSA, containing a
   Links with Backup Paths TLV, is used to flood the flooding topology
   with backup paths from the leader of an area to all the other nodes
   in the area.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            LS age             |     Options   | LS Type = 10  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |FTBP-Type(TBDa)|                   Instance ID                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Advertising Router                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      LS Sequence Number                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         LS checksum           |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                Links with Backup Paths TLV                    ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        OSPFv2: Flooding Topology with Backup Paths (FTBP) Opaque LSA


   For OSPFv3, an area scope LSA of a new LSA function code (TBDb),
   containing a Links with Backup Paths TLV, is used to flood the
   flooding topology with backup paths from the leader of an area to all
   the other nodes in the area.








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        0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            LS age             |1|0|1|     FTBP-LSA (TBDb)     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Link State ID                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Advertising Router                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      LS Sequence Number                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         LS checksum           |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                  Links with Backup Paths TLV                  ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            OSPFv3: Flooding Topology with Backup Paths (FTBP) LSA


6.2.3.  Message for Incremental Changes

   For adding some links to the flooding topology, we define a new TLV
   called Add Links TLVs of the following format.  When some new links
   are added to the flooding topology, the leader may not flood the
   whole flooding topology with the new links to all the other nodes.
   It may just flood these new links.  After receiving these new links,
   each of the other nodes adds these new links into the existing
   flooding topology.  When the leader floods the whole flooding
   topology with the new links to all the other nodes, it removes the
   LSA for the new links.  When removing the LSA for these new links,
   each of the other nodes does not update the flooding topology (i.e.,
   does not remove these links from the flooding topology).

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      ADDLK-TLV-Type (TBDc)    |          TLV-Length           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 1 to its adjacent Nodes)       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 2 to its adjacent Nodes)       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                                                               :
    :                                                               :
                             Add Links TLV






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   For deleting some links from the flooding topology, we define a new
   TLV called Delete Links TLVs of the following format.  When some old
   links are removed from the flooding topology, the leader may not
   flood the whole flooding topology without the old links to all the
   other nodes.  It may just flood these old links.  After receiving
   these old links, each of the other nodes deletes these old links from
   the existing flooding topology.  When the leader floods the whole
   flooding topology without the old links to all the other nodes, it
   removes the LSA for the old links.  When removing the LSA for these
   old links, each of the other nodes does not update the flooding
   topology (i.e., does not add these links into the flooding topology).

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      DELLK-TLV-Type (TBDd)    |          TLV-Length           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 1 to its adjacent Nodes)       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 2 to its adjacent Nodes)       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                                                               :
    :                                                               :
                             Delete Links TLV


   The Add Links TLVs and Delete Links TLVs should be in a separate LSA
   instance.  The LSA can be a Flooding Topology LSA defined above.
   Alternatively, we may define a new LSA for these TLVs.

6.2.4.  Leaders Selection

   The leader or Designated Router (DR) selection for a broadcast link
   is about selecting two leaders: a DR and Backup DR.  This is
   generalized to select two or more leaders for an area: the primary/
   first leader (or leader for short), the secondary leader, the third
   leader and so on.

   A new TLV is defined to include the information on flooding reduction
   of a node, which is called Flooding Reduction Information TLV or
   Information TLV for short.  This TLV is generated by every node that
   supports flooding reduction in general.  Every node originates a RI
   LSA with a Flooding Reduction Information TLV containing its priority
   to become a leader.  The format of the TLV is as follows.







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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      INFO-TLV-Type (TBDe)     |          TLV-Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Priority   |             Reserved (MUST be zero)           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    sub TLVs (optional)                        ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Flooding Reduction Information TLV


   A Priority field of eight bits is defined in the TLV to indicate the
   priority of the node originating the TLV to become the leader node in
   central mode.

   A sub-TLV called leaders sub-TLV is defined.  It has the following
   format.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      LEADS-TLV-Type (TBDf)    |          TLV-Length           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    1st Leader Node/Router ID                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    2nd Leader Node/Router ID                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                                                               ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    nth Leader Node/Router ID                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             Leaders sub-TLV


   When a node selects itself as a leader, it originates a RI LSA
   containing the leader in a leaders sub-TLV.

   After the first leader node is down, the other leaders will be
   promoted.  The secondary leader becomes the first leader, the third
   leader becomes the secondary leader, and so on.  When a node selects
   itself as the n-th leader, it originates a RI LSA with a Leaders sub-
   TLV containing n leaders.






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7.  Extensions to IS-IS

   The extensions to IS-IS is similar to OSPF.

7.1.  Extensions for Operations

   A new TLV for operations is defined in IS-IS LSP.  It has the
   following format and contains the same contents as the Flooding
   Reduction Instruction TLV defined in OSPF RI LSA.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |INST-Type(TBDi1|    Length     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  OP | MOD |   Algorithm   |    Reserved (MUST be zero)  |  NL |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    sub TLVs (optional)                        ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     IS-IS Flooding Reduction Instruction TLV


7.2.  Extensions for Centralized Mode

7.2.1.  TLV for Flooding Topology

   A new TLV for the encodings of the links in the flooding topology is
   defined.  It has the following format and contains the same contents
   as the Flooding Topology Links TLV defined in OSPF Flooding Topology
   Opaque LSA.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |FTL-Type(TBDi2)|    Length     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 1 to its adjacent Nodes)       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 2 to its adjacent Nodes)       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                                                               :
    :                                                               :
                         IS-IS Flooding Topology Links TLV







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7.2.2.  Encodings for Backup Paths

7.2.2.1.  TLVs for Backup Paths

   For flooding backup paths separately, we define two TLVs: IS-IS Node
   Backup Paths TLV and IS-IS Link Backup Path TLV.  The former has the
   following format and contains the same contents as Node Backup Paths
   TLV in OSPF.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |NBP-Type(TBDi3)|    Length     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Node1 Index Enc|  Variable bits
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                 Node1 backup paths encoding                   :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Node2 Index Enc|  Variable bits
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                 Node2 backup paths encoding                   :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                                                             //
                         IS-IS Node Backup Paths TLV


   The latter has the following format and contains the same contents as
   Link Backup Paths TLV in OSPF.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |LBP-Type(TBDi4)|    Length     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Link1 Index Enc|  Variable bits
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                 Link1 backup paths encoding                   :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Link2 Index Enc|  Variable bits
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                 Link2 backup paths encoding                   :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                                                             //
                         IS-IS Link Backup Paths TLV







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7.2.2.2.  Backup Paths in Links TLV

   A new TLV is defined to integrate the backup paths with the links on
   the flooding topology.  It has the following format and contains the
   same contents as the Links with Backup Paths TLV in OSPF.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |LSB-Type(TBDi5)|    Length   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :          Links from Node 1 with backup paths encoding         :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :          Links from Node 2 with backup paths encoding         :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                                                               :
    :                                                               :
                      IS-IS Links with Backup Paths TLV


7.2.3.  TLVs for Incremental Changes

   Similar to Add Links TLV in OSPF, a new TLV called IS-IS Add Links
   TLV is defined.  It has the following format and contains the same
   contents as Add Links TLV in OSPF.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |ADDL-Type(TBDi6|     Length    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 1 to its adjacent Nodes)       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 2 to its adjacent Nodes)       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                                                               :
    :                                                               :
                          IS-IS Add Links TLV


   Similar to Delete Links TLV in OSPF, a new TLV called IS-IS Delete
   Links TLV is defined.  It has the following format and contains the
   same contents as Delete Links TLV in OSPF.








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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |DELL-Type(TBDi7|      Length   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 1 to its adjacent Nodes)         ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~           Links Encoding (Node 2 to its adjacent Nodes)         ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                                                               :
    :                                                               :
                          IS-IS Delete Links TLV


7.2.4.  Leaders Selection

   Similar to Flooding Reduction Information TLV in OSPF, a new TLV
   called IS-IS Flooding Reduction Information TLV is defined.  It has
   the following format and contains the same contents as Flooding
   Reduction Information TLV in OSPF.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |INF-Type(TBDi8)|     Length    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Priority   |             Reserved (MUST be zero)           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    sub TLVs (optional)                        ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    IS-IS Flooding Reduction Information TLV


8.  Flooding Behavior

   This section describes the revised flooding behavior for a node
   having at least one link on the flooding topology.  The revised
   flooding procedure MUST flood an LS to every node in the network in
   any case, as the standard flooding procedure does.

8.1.  Nodes Perform Flooding Reduction without Failure

8.1.1.  Receiving an LS

   When a node receives a newer LS that is not originated by itself from
   one of its interfaces, it floods the LS only to all the other
   interfaces that are on the flooding topology.



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   When the LS is received from an interface on the flooding topology,
   it is flooded only to all the other interfaces that are on the
   flooding topology.  When the LS is received on an interface that is
   not on the flooding topology, it is also flooded only to all the
   other interfaces that are on the flooding topology.

   In any case, the LS must not be transmitted back to the receiving
   interface.

   Note before forwarding a received LS, the node would do the normal
   processing as usual.

8.1.2.  Originating an LS

   When a node originates an LS, it floods the LS to its interfaces on
   the flooding topology if the LS is a refresh LS (i.e., there is no
   significant change in the LS comparing to the previous LS); otherwise
   (i.e., there are significant changes in the LS), it floods the LS to
   all its interfaces.  Choosing flooding the LS with significant
   changes to all the interfaces instead of limiting to the interfaces
   on the flooding topology would speed up the distribution of the
   significant link state changes.

8.1.3.  Establishing Adjacencies

   Adjacencies being established can be classified into two categories:
   adjacencies to new nodes and adjacencies to existing nodes.

8.1.3.1.  Adjacency to New Node

   An adjacency to a new node is an adjacency between a node (say node
   A) on the flooding topology and the new node (say node Y) which is
   not on the flooding topology.  There is not any adjacency between
   node Y and a node in the network area.

   When new node Y is up and connected to node A, node A assumes that
   node Y and the link between node Y and node A are on the flooding
   topology until a new flooding topology is computed and built.  Node A
   may determine whether node Y is a new node through checking if node Y
   is reachable or on the flooding topology.

   The procedure for establishing the adjacency between node A and node
   Y is the existing normal procedure unchanged.  After the status of
   the adjacency reaches to Exchange or Full, node A sends node Y every
   new or updated LS that node A receives or originates.






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8.1.3.2.  Adjacency to Existing Node

   An adjacency to an existing node is an adjacency between a node (say
   node A) on the flooding topology and the existing node (say node X)
   which exists on the flooding topology.  There are some adjacencies
   between node X and some nodes in the network area.

   When existing node X is connected to node A after a link between node
   X and node A is up, node A assumes that the link connecting node A
   and node X is not on the flooding topology until a new flooding
   topology is computed and built.  Node A may determine whether node X
   is an existing node through checking if node X is reachable or on the
   flooding topology.

   The procedure for establishing the adjacency between node A and node
   X is the existing normal procedure unchanged.  Node A does not send
   node X any new or updated LS that node A receives or originates even
   after the status of the adjacency reaches to Exchange or Full.

8.2.  An Exception Case

   During an LS flooding, one or multiple link and node failures may
   happen.  Some failures do not split the flooding topology, thus do
   not affect the flooding behavior.  For example, multiple failures of
   the links not on the flooding topology do not split the flooding
   topology and do not affect the flooding behavior.  The sections below
   focus on the failures that may split the flooding topology.

8.2.1.  A Critical Failure

   For a link failure, if the link is a critical link on the flooding
   topology, then the LS is flooded through a backup path for the link
   and the remaining flooding topology until a new flooding topology is
   computed and built; otherwise, the flooding behavior in Section 8.1
   follows.

   Similarly, for a node failure, if the node is a critical node on the
   flooding topology, then the LS is flooded through backup paths for
   the node and the remaining flooding topology until a new flooding
   topology is computed and built; otherwise, the flooding behavior in
   Section 8.1 follows.

8.2.2.  Multiple Failures

   For multiple link failures, if the number of the failed links on the
   flooding topology is greater than or equal to two, then the LS is
   flooded through a backup path for each of the failed links on the
   flooding topology and the remaining flooding topology until a new



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   flooding topology is computed and built; otherwise, the flooding
   behavior in Section 8.1 follows.

   If all the backup paths for some of the failed links are broken by
   some failures, the LS is flooded to all interfaces (except where it
   is received from) until a new flooding topology is computed and
   built.

   For multiple node failures, the LS is flooded through the backup
   paths for each of the failed nodes and the remaining flooding
   topology until a new flooding topology is computed and built;
   otherwise, the flooding behavior in Section 8.1 follows.

   If the backup paths for some of the failed nodes are broken by some
   failures, the LS is flooded to all interfaces (except where it is
   received from) until a new flooding topology is computed and built.

   Note that if it can be quickly determined that the flooding topology
   is not split by the failures, the flooding behavior in Section 8.1
   may follow.

9.  Security Considerations

   This document does not introduce any security issue.

10.  IANA Considerations

10.1.  OSPFv2

   Under Registry Name: OSPF Router Information (RI) TLVs [RFC7770],
   IANA is requested to assign two new TLV values for OSPF flooding
   reduction as follows:

     +===============+==================+=====================+
     |  TLV Value    |    TLV Name      |    reference        |
     +===============+==================+=====================+
     |      11       | Instruction TLV  |    This document    |
     +---------------+------------------+---------------------+
     |      12       | Information TLV  |    This document    |
     +---------------+------------------+---------------------+

   Under the registry name "Opaque Link-State Advertisements (LSA)
   Option Types" [RFC5250], IANA is requested to assign new Opaque Type
   registry values for FT LSA, BP LSA, FTBP LSA as follows:







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     +====================+===============+=======================+
     |  Registry Value    |  Opaque Type  |    reference          |
     +====================+===============+=======================+
     |         10         |    FT LSA     |    This document      |
     +--------------------+---------------+-----------------------+
     |         11         |    BP LSA     |    This document      |
     +--------------------+---------------+-----------------------+
     |         12         |   FTBP LSA    |    This document      |
     +--------------------+---------------+-----------------------+

   IANA is requested to create and maintain new registries:

    o OSPFv2 FT LSA TLVs

   Initial values for the registry are given below.  The future
   assignments are to be made through IETF Review [RFC5226].

       Value         OSPFv2 FT LSA TLV Name     Definition
       -----         -----------------------    ----------
       0             Reserved
       1             FT Links TLV               see Section 6.2.1
       2-32767       Unassigned
       32768-65535   Reserved

    o OSPFv2 BP LSA TLVs

   Initial values for the registry are given below.  The future
   assignments are to be made through IETF Review [RFC5226].

       Value         OSPFv2 TBPLSA TLV Name     Definition
       -----         -----------------------    ----------
       0             Reserved
       1             Node Backup Paths TLV      see Section 6.2.2
       2             Link Backup Paths TLV      see Section 6.2.2
       3-32767       Unassigned
       32768-65535   Reserved

    o OSPFv2 FTBP LSA TLVs

   Initial values for the registry are given below.  The future
   assignments are to be made through IETF Review [RFC5226].

       Value         OSPFv2 FTBP LSA TLV Name       Definition
       -----         ------------------------       ----------
       0             Reserved
       1             Links with Backup Paths TLV   see Section 6.2.2
       2-32767       Unassigned
       32768-65535   Reserved



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10.2.  OSPFv3

   Under the registry name "OSPFv3 LSA Function Codes", IANA is
   requested to assign new registry values for FT LSA, BP LSA, FTBP LSA
   as follows:

     +===========+==========================+=======================+
     |  Value    |  LSA Function Code Name  |    reference          |
     +======================================+=======================+
     |    16     |         FT LSA           |    This document      |
     +-----------+--------------------------+-----------------------+
     |    17     |         BP LSA           |    This document      |
     +-----------+--------------------------+-----------------------+
     |    18     |         FTBP LSA         |    This document      |
     +-----------+--------------------------+-----------------------+

   IANA is requested to create and maintain new registries:

    o OSPFv3 FT LSA TLVs

   Initial values for the registry are given below.  The future
   assignments are to be made through IETF Review [RFC5226].

       Value         OSPFv3 FT LSA TLV Name     Definition
       -----         -----------------------    ----------
       0             Reserved
       1             FT Links TLV               see Section 6.2.1
       2-32767       Unassigned
       32768-65535   Reserved

    o OSPFv3 BP LSA TLVs

   Initial values for the registry are given below.  The future
   assignments are to be made through IETF Review [RFC5226].

       Value         OSPFv3 TBPLSA TLV Name     Definition
       -----         -----------------------    ----------
       0             Reserved
       1             Node Backup Paths TLV      see Section 6.2.2
       2             Link Backup Paths TLV      see Section 6.2.2
       3-32767       Unassigned
       32768-65535   Reserved

    o OSPFv3 FTBP LSA TLVs

   Initial values for the registry are given below.  The future
   assignments are to be made through IETF Review [RFC5226].




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       Value         OSPFv3 FTBP LSA TLV Name       Definition
       -----         ------------------------       ----------
       0             Reserved
       1             Links with Backup Paths TLV   see Section 6.2.2
       2-32767       Unassigned
       32768-65535   Reserved

10.3.  IS-IS

   Under Registry Name: IS-IS TLV Codepoints, IANA is requested to
   assign new TLV values for IS-IS flooding reduction as follows:

       Value     TLV Name                      Definition
       -----    ------------------------       ----------
        151      FT Links TLV                   see Section 7.2.1
        152      Node Backup Paths TLV          see Section 7.2.2
        153      Link Backup Paths TLV          see Section 7.2.2
        154      Links with Backup Paths TLV    see Section 7.2.2
        155      Add Links TLV                  see Section 7.2.3
        156      Delete Links TLV               see Section 7.2.3
        157      Instruction TLV                see Section 7.1
        158      Information TLV                see Section 7.2.4

11.  Acknowledgements

   The authors would like to thank Acee Lindem, Zhibo Hu, Robin Li,
   Stephane Litkowski and Alvaro Retana for their valuable suggestions
   and comments on this draft.

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC5250]  Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
              OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250,
              July 2008, <https://www.rfc-editor.org/info/rfc5250>.






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   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC7770]  Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
              S. Shaffer, "Extensions to OSPF for Advertising Optional
              Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
              February 2016, <https://www.rfc-editor.org/info/rfc7770>.

12.2.  Informative References

   [I-D.li-dynamic-flooding]
              Li, T. and P. Psenak, "Dynamic Flooding on Dense Graphs",
              draft-li-dynamic-flooding-05 (work in progress), June
              2018.

   [I-D.shen-isis-spine-leaf-ext]
              Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS
              Routing for Spine-Leaf Topology", draft-shen-isis-spine-
              leaf-ext-06 (work in progress), June 2018.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.

Appendix A.  Algorithms to Build Flooding Topology

   There are many algorithms to build a flooding topology.  A simple and
   efficient one is briefed below.

   o  Select a node R according to a rule such as the node with the
      biggest/smallest node ID;

   o  Build a tree using R as root of the tree (details below); and then

   o  Connect k (k>=0) leaves to the tree to have a flooding topology
      (details follow).

A.1.  Algorithms to Build Tree without Considering Others

   An algorithm for building a tree from node R as root starts with a
   candidate queue Cq containing R and an empty flooding topology Ft:

   1.  Remove the first node A from Cq and add A into Ft

   2.  If Cq is empty, then return with Ft




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   3.  Suppose that node Xi (i = 1, 2, ..., n) is connected to node A
       and not in Ft and X1, X2, ..., Xn are in a special order.  For
       example, X1, X2, ..., Xn are ordered by the cost of the link
       between A and Xi.  The cost of the link between A and Xi is less
       than the cost of the link between A and Xj (j = i + 1).  If two
       costs are the same, Xi's ID is less than Xj's ID.  In another
       example, X1, X2, ..., Xn are ordered by their IDs.  If they are
       not ordered, then make them in the order.

   4.  Add Xi (i = 1, 2, ..., n) into the end of Cq, goto step 1.

   Another algorithm for building a tree from node R as root starts with
   a candidate queue Cq containing R and an empty flooding topology Ft:

   1.  Remove the first node A from Cq and add A into Ft

   2.  If Cq is empty, then return with Ft

   3.  Suppose that node Xi (i = 1, 2, ..., n) is connected to node A
       and not in Ft and X1, X2, ..., Xn are in a special order.  For
       example, X1, X2, ..., Xn are ordered by the cost of the link
       between A and Xi.  The cost of the link between A and Xi is less
       than the cost of the link between A and Xj (j = i + 1).  If two
       costs are the same, Xi's ID is less than Xj's ID.  In another
       example, X1, X2, ..., Xn are ordered by their IDs.  If they are
       not ordered, then make them in the order.

   4.  Add Xi (i = 1, 2, ..., n) into the front of Cq and goto step 1.

   A third algorithm for building a tree from node R as root starts with
   a candidate list Cq containing R associated with cost 0 and an empty
   flooding topology Ft:

   1.  Remove the first node A from Cq and add A into Ft

   2.  If all the nodes are on Ft, then return with Ft

   3.  Suppose that node A is associated with a cost Ca which is the
       cost from root R to node A, node Xi (i = 1, 2, ..., n) is
       connected to node A and not in Ft and the cost of the link
       between A and Xi is LCi (i=1, 2, ..., n).  Compute Ci = Ca + LCi,
       check if Xi is in Cq and if Cxi (cost from R to Xi) < Ci.  If Xi
       is not in Cq, then add Xi with cost Ci into Cq; If Xi is in Cq,
       then If Cxi > Ci then replace Xi with cost Cxi by Xi with Ci in
       Cq; If Cxi == Ci then add Xi with cost Ci into Cq.

   4.  Make sure Cq is in a special order.  Suppose that Ai (i=1, 2,
       ..., m) are the nodes in Cq, Cai is the cost associated with Ai,



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       and IDi is the ID of Ai.  One order is that for any k = 1, 2,
       ..., m-1, Cak < Caj (j = k+1) or Cak = Caj and IDk < IDj.  Goto
       step 1.

A.2.  Algorithms to Build Tree Considering Others

   An algorithm for building a tree from node R as root with
   consideration of others's support for flooding reduction starts with
   a candidate queue Cq containing R associated with previous hop PH=0
   and an empty flooding topology Ft:

   1.  Remove the first node A that supports flooding reduction from the
       candidate queue Cq if there is such a node A; otherwise (i.e., if
       there is not such node A in Cq), then remove the first node A
       from Cq.  Add A into the flooding topology Ft.

   2.  If Cq is empty or all nodes are on Ft, then return with Ft

   3.  Suppose that node Xi (i = 1, 2, ..., n) is connected to node A
       and not in the flooding topology Ft and X1, X2, ..., Xn are in a
       special order considering whether some of them that support
       flooding reduction (.  For example, X1, X2, ..., Xn are ordered
       by the cost of the link between A and Xi.  The cost of the link
       between A and Xi is less than that of the link between A and Xj
       (j = i + 1).  If two costs are the same, Xi's ID is less than
       Xj's ID.  The cost of a link is redefined such that 1) the cost
       of a link between A and Xi both support flooding reduction is
       much less than the cost of any link between A and Xk where Xk
       with F=0; 2) the real metric of a link between A and Xi and the
       real metric of a link between A and Xk are used as their costs
       for determining the order of Xi and Xk if they all (i.e., A, Xi
       and Xk) support flooding reduction or none of Xi and Xk support
       flooding reduction.

   4.  Add Xi (i = 1, 2, ..., n) associated with previous hop PH=A into
       the end of the candidate queue Cq, and goto step 1.

   Another algorithm for building a tree from node R as root with
   consideration of others' support for flooding reduction starts with a
   candidate queue Cq containing R associated with previous hop PH=0 and
   an empty flooding topology Ft:

   1.  Remove the first node A that supports flooding reduction from the
       candidate queue Cq if there is such a node A; otherwise (i.e., if
       there is not such node A in Cq), then remove the first node A
       from Cq.  Add A into the flooding topology Ft.

   2.  If Cq is empty or all nodes are on Ft, then return with Ft.



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   3.  Suppose that node Xi (i = 1, 2, ..., n) is connected to node A
       and not in the flooding topology Ft and X1, X2, ..., Xn are in a
       special order considering whether some of them support flooding
       reduction.  For example, X1, X2, ..., Xn are ordered by the cost
       of the link between A and Xi.  The cost of the link between A and
       Xi is less than the cost of the link between A and Xj (j = i +
       1).  If two costs are the same, Xi's ID is less than Xj's ID.
       The cost of a link is redefined such that 1) the cost of a link
       between A and Xi both support flooding reduction is much less
       than the cost of any link between A and Xk where Xk does not
       support flooding reduction; 2) the real metric of a link between
       A and Xi and the real metric of a link between A and Xk are used
       as their costs for determining the order of Xi and Xk if they all
       (i.e., A, Xi and Xk) support flooding reduction or none of Xi and
       Xk supports flooding reduction.

   4.  Add Xi (i = 1, 2, ..., n) associated with previous hop PH=A into
       the front of the candidate queue Cq, and goto step 1.

   A third algorithm for building a tree from node R as root with
   consideration of others' support for flooding reduction (using flag F
   = 1 for support, and F = 0 for not support in the following) starts
   with a candidate list Cq containing R associated with low order cost
   Lc=0, high order cost Hc=0 and previous hop ID PH=0, and an empty
   flooding topology Ft:

   1.  Remove the first node A from Cq and add A into Ft.

   2.  If all the nodes are on Ft, then return with Ft

   3.  Suppose that node A is associated with a cost Ca which is the
       cost from root R to node A, node Xi (i = 1, 2, ..., n) is
       connected to node A and not in Ft and the cost of the link
       between A and Xi is LCi (i=1, 2, ..., n).  Compute Ci = Ca + LCi,
       check if Xi is in Cq and if Cxi (cost from R to Xi) < Ci.  If Xi
       is not in Cq, then add Xi with cost Ci into Cq; If Xi is in Cq,
       then If Cxi > Ci then replace Xi with cost Cxi by Xi with Ci in
       Cq; If Cxi == Ci then add Xi with cost Ci into Cq.

   4.  Suppose that node A is associated with a low order cost LCa which
       is the low order cost from root R to node A and a high order cost
       HCa which is the high order cost from R to A, node Xi (i = 1, 2,
       ..., n) is connected to node A and not in the flooding topology
       Ft and the real cost of the link between A and Xi is Ci (i=1, 2,
       ..., n).  Compute LCxi and HCxi: LCxi = LCa + Ci if both A and Xi
       have flag F set to one, otherwise LCxi = LCa HCxi = HCa + Ci if A
       or Xi does not have flag F set to one, otherwise HCxi = HCa If Xi
       is not in Cq, then add Xi associated with LCxi, HCxi and PH = A



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       into Cq; If Xi associated with LCxi' and HCxi' and PHxi' is in
       Cq, then If HCxi' > HCxi then replace Xi with HCxi', LCxi' and
       PHxi' by Xi with HCxi, LCxi and PH=A in Cq; otherwise (i.e.,
       HCxi' == HCxi) if LCxi' > LCxi , then replace Xi with HCxi',
       LCxi' and PHxi' by Xi with HCxi, LCxi and PH=A in Cq; otherwise
       (i.e., HCxi' == HCxi and LCxi' == LCxi) if PHxi' > PH, then
       replace Xi with HCxi', LCxi' and PHxi' by Xi with HCxi, LCxi and
       PH=A in Cq.

   5.  Make sure Cq is in a special order.  Suppose that Ai (i=1, 2,
       ..., m) are the nodes in Cq, HCai and LCai are low order cost and
       high order cost associated with Ai, and IDi is the ID of Ai.  One
       order is that for any k = 1, 2, ..., m-1, HCak < HCaj (j = k+1)
       or HCak = HCaj and LCak < LCaj or HCak = HCaj and LCak = LCaj and
       IDk < IDj.  Goto step 1.

A.3.  Connecting Leaves

   Suppose that we have a flooding topology Ft built by one of the
   algorithms described above.  Ft is like a tree.  We may connect k (k
   >=0) leaves to the tree to have a enhanced flooding topology with
   more connectivity.

   Suppose that there are m (0 < m) leaves directly connected to a node
   X on the flooding topology Ft.  Select k (k <= m) leaves through
   using a deterministic algorithm or rule.  One algorithm or rule is to
   select k leaves that have smaller or larger IDs (i.e., the IDs of
   these k leaves are smaller/bigger than the IDs of the other leaves
   directly connected to node X).  Since every node has a unique ID,
   selecting k leaves with smaller or larger IDs is deterministic.

   If k = 1, the leaf selected has the smallest/largest node ID among
   the IDs of all the leaves directly connected to node X.

   For a selected leaf L directly connected to a node N in the flooding
   topology Ft, select a connection/adjacency to another node from node
   L in Ft through using a deterministic algorithm or rule.

   Suppose that leaf node L is directly connected to nodes Ni (i =
   1,2,...,s) in the flooding topology Ft via adjacencies and node Ni is
   not node N, IDi is the ID of node Ni, and Hi (i = 1,2,...,s) is the
   number of hops from node L to node Ni in the flooding topology Ft.

   One Algorithm or rule is to select the connection to node Nj (1 <= j
   <= s) such that Hj is the largest among H1, H2, ..., Hs.  If there is
   another node Na ( 1 <= a <= s) and Hj = Ha, then select the one with
   smaller (or larger) node ID.  That is that if Hj == Ha and IDj < IDa
   then select the connection to Nj for selecting the one with smaller



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   node ID (or if Hj == Ha and IDj < IDa then select the connection to
   Na for selecting the one with larger node ID).

   Suppose that the number of connections in total between leaves
   selected and the nodes in the flooding topology Ft to be added is
   NLc.  We may have a limit to NLc.

Authors' Addresses

   Huaimo Chen
   Huawei Technologies

   Email: huaimo.chen@huawei.com


   Dean Cheng
   Huawei Technologies

   Email: dean.cheng@huawei.com


   Mehmet Toy
   Verizon
   USA

   Email: mehmet.toy@verizon.com


   Yi Yang
   IBM
   Cary, NC
   United States of America

   Email: yyietf@gmail.com

















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